This invention relates generally to methods and circuitry for driving matrix displays and more particularly to ferroelectric liquid crystal displays.
The idea to use ferroelectric liquid crystals in display devices was proposed by N. Clark and S. Lagerwall, U.S. Pat. No. 4,367,924. The materials used in these devices are smectic C or H liquid crystals. These are material wherein the molecules lie in planes and the molecules are tilted within the planes and rotate in a conical locus about the planar normal. These materials exhibit bi-stability, that is, can be switched between two stable states by reversing the polarity of an externally applied electric field, to make a bi-stable electro-optical device. Chiral smectic C materials (SmC*) further possess a permanent dipole moment that lies in the smectic planes and is normal to the long molecular axis. This dipole moment couples strongly with applied electric fields and allows the rotation angle of the molecules about planar normal to be controlled by applying an electric field parallel to the smectic planes.
A ferroelectric liquid crystal cell is formed by confining a thin layer of the material between two layers of glass. The ferroelectric liquid crystal molecules at the surface of the glass are constrained to be flat against the surface. The director field can thus adopt two uniform states. Electrodes in the form of transparent conductive stripes are applied to the interior surfaces of the sheets of glass to complete the liquid crystal cell. The electrodes form M columns on one surface and N transverse rows on the opposing surface. The intersections of the rows and columns define pixels in an M.times.N matrix. The pixels are set to a desired display state by applying waveforms to the M rows called "strobe waveforms" and applying waveforms to the N columns called "data waveforms," each consisting of select or non-select data waveforms.
The field applied by the electrodes will be parallel to the smectic planes if the planes are oriented normal to the surfaces of the glass. Clark et al. describe a method for attaining this orientation. The resultant structure provides a liquid crystal display device comprising a matrix of N.times.M pixels that are switchable between at least two display states. By use of polarizers, the states can be made optically distinguishable.
The ability to make operable ferroelectric liquid crystal matrix display devices work has been demonstrated. A number of problems arise, however, in trying to design a practical, commercially-applicable ferroelectric liquid crystal display. In practice, building a device which is defect free and has "good" bi-stability is a problem. Among the most important considerations is how to drive the display. The forms of applied strobe and data waveforms proposed to date have not provided satisfactory matrix display operation.
Most liquid crystals that have been employed in conventional practice for commercial liquid crystal display devices are twisted nematic-type liquid crystals. Nematic liquid crystals have a positive dielectric anisotropy. Twisted nematic liquid crystal displays can be effectively driven by applying static DC signals to the electrodes. These signal levels are conventionally provided by a fixed resistive-capacitive voltage divider. A typical driver circuit for twisted nematic-type liquid crystal displays is the MSM5260 GSK Model 80 bit LCD dot matrix driver and associated circuitry, manufactured by OKI Electric Industry Co., Ltd., of Sunnyvale, California.
Such a driver typically includes a serial-to-parallel N-bit shift register into which display data (for the column or X driver) and line select data (for the row or Y driver) are input. The shift register outputs the data in parallel to a latch, which passes the data to a N-wide 4:1 analog multiplexer array. Each multiplexer has two select inputs, one of which (FR) is common to all, the other (DATA) connected to receive data from the latch. The multiplexers each have four voltage level inputs in common. The static DC levels are applied to these inputs. These circuits are powered from a supply voltage V.sub.DD (typically +5 VDC) and V.sub.SSH (variable according to temperature, LCD type, and matrix size). In the MSM 5260, V.sub.DD also serves as one of the static DC reference voltage inputs (V1). The FR input is conventionally inverted in a 50% duty cycle, typically once per frame but, in some implementations, as often as once per line, to select between pairs of the static voltage inputs.
For a number of reasons, conventional driver circuitry of the type used for twisted nematic liquid crystal displays is not satisfactory for ferroelectric liquid crystal displays. In ferroelectric displays, it is necessary to have bi-stable latching, that further has some threshold. In an actual matrix device there will always be some background voltages present at every pixel. It is important that these voltages not lower the contrast or change the state of a pixel but, on the other hand, that only a slightly greater voltage suffices to change the state of the pixel.
Another concern has to do with the nature of the electrical waveforms to be applied to the ferroelectric display device. The response of the device is dependent on the polarity of the applied voltage and on the product of the voltage and the duration of the applied pulse. In a ferroelectric device, a "+" polarity pulse switches a pixel to one state and a "-" pulse to the other. There is a threshold associated with this process that is, roughly speaking, related to the area under the pulse. Sub-threshold pulses (those that do not cause switching), however, can disturb the pixel. That is, while the pulse is applied, the director configuration distorts. In a matrix device, strobe waveforms will be applied to transparent conductive rows on one cell surface, and data waveforms to columns on the other surface. Waveforms applied to the rows and columns of the device produce a difference waveform from a particular row and column at each pixel. This difference waveform needs to be +V or -V during each time frame to set the selected pixels in a desired state. The rest of the time, the difference waveform should do nothing to the pixels. The data signals should affect only the strobed row, setting the pixels in that row ON or OFF while leaving the pixels in non-strobed rows unchanged. The rows are strobed sequentially so that a pixel sees the resultant of a strobe waveform and the data waveform for 1/N of the time (N=total number of matrix lines) and sees the data voltage alone for the rest of the time. It turns out that a continuously-applied DC voltage, even if it is quite small, can affect the pixels' state. Thus, another requirement for ferroelectric liquid crystal display devices is that the background voltage be "AC-like."
Lagerwall et al., in the Proceedings of the 1985 International Display Research Conference, IEEE, pages 213-221 (1985), have proposed a five-phase bipolar drive waveform to meet these needs (see Lagerwall et al., FIG. 7). It requires that each row be addressed five times in a frame cycle: once to switch pixels in a row "up"; once to cancel the DC component of the first switching pulse; once to switch pixels in the row "down"; once to cancel the DC component of the third switching pulse; and once more to further increase the AC nature of the resultant waveform. Looking at the resultant waveforms, it can be seen that an "up" signal really goes "down-up" and a "down" signal really goes "up-down." Also, the data voltage waveform has "+" pulses followed by "-" pulses and vice versa to minimize the effect of the "background" voltage each pixel sees by making it more "AC" like.
There are two objectionable features to the Lagerwall et al. waveform. One problem is that the black area of the screen may tend to flicker if the refresh rate is slow. This is because black pixels will momentarily flash white during each refresh cycle. The other problem is that the refresh cycle will tend to take a lot of time because each row must be addressed with a five-phase sequence, where each phase interval is roughly equal to the switching seed of a pixel. Thus, if a pixel can be switched in 100 microsecs, 500 microsecs per line is required for every frame update of matrix device.
More recent results of an attempt to produce a ferroelectric matrix display device were presented in the paper, "Optimum Bias Condition for Multiplex Driving of the Chiral Smectic C LCD", Japan Display '86, pp. 460-462 by S. Shimoda et al. from Seiko Instruments. FIG. 1 gives the resultant voltages that an ON pixel sees (pixel-1) and that an OFF pixel sees (pixel-2). The waveform that each pixel sees is an alternately-polarized squarewave, is symmetric about a zero DC voltage level, has a period T, and uses three voltage magnitudes V.sub.S &gt;V.sub.N &gt;V.sub.H. Only magnitude V.sub.S is intended to change the display state of the pixels, the state being determined by the polarity of the last-occurring pulse of magnitude V.sub.S (-V.sub.S =ON, +V.sub.S =OFF). In practice, however, this type of waveform produces very low display contrast, as is apparent from the transmission characteristics shown in FIG. 3 of the paper. It can be seen from the bottom half of FIG. 3 that the light transmission of a pixel that is ON (left half) is not much different from a pixel in the OFF state (right half).
Aside from the fact that the ferroelectric display drive waveform of Shimoda et al. provides a very low contrast ratio between the ON and OFF states, the waveform has other undesirable features. One is the that the pixels will tend to flicker when switched to OFF or black, especially if the addressing time of the display is greater than 20 ms. A second is that the pixels of a given column will have an average light transmission that is dependent on the state of the rest of the pixels in the column. Another undesirable feature is the time required to completely address the proposed matrix display device. In large displays, it would clearly be of use to reduce the time required to address the display by using some other waveform.
Other driving methods have been proposed for ferroelectric liquid crystal displays but these do not adequately address the concerns outlined above. U.S. Pat. No. 4,638,310 to Ayliffe discloses a method for addressing a ferroelectric liquid crystal matrix display which applies strobing pulses serially to the electrodes on one side of the display and applies balanced bipolar data pulses in parallel to the electrodes on the other side. The data pulses are twice as long as the strobing pulses.
U.S. Pat. No. 4,645,303 to Sekiya et al. discloses a liquid crystal matrix display panel drive method that appears to be directed to driving twisted nematic-type LCDs. It discusses how the effective RMS value (of concern in TN-type LCDs which are not bistable) of the drive voltage applied to the display element can be affected by the states of other elements driven by a common conductor. It proposes a drive scheme which seeks to eliminate pattern-dependent contrast effects in large-area displays which result from matrix conductor resistance and display element capacitance, and flickering if it is ensured that the duration of each cycle interval is shorter than the response time of the liquid crystal. Thus, Sekiya, et al. does not address the principal problems attendant to driving ferroelectric matrix displays.
U.S. Pat. No. 4,548,476 to Kaneko proposes a time-sharing driving method that employs a two-level drive scheme. This patent does not address the problems of complex multiplexing, as set out above, in a large (i.e., M.times.N where both M and N are typically greater than 2.sup.6) matrix display.
U.S. Pat. No. 4,508,429 to Nagae et al describes a method for controlling a switch for producing a two-level waveform (+V.sub.p and -V.sub.p) and DC-offset variations thereof to switch and periodically refresh the state of the liquid crystal, in a scheme that provides an average voltage level of zero. The method does not provide for a multitude of levels and durations, however, because it does not involve bi-stable liquid crystals. Also, the form of drive circuit disclosed would, apparently have to be duplicated for each row and column of the matrix. This is not a practical solution for a large matrix display.
In summary, the lack of "good" bi-stability of the ferroelectric display elements and of desirable characteristics of the applied waveforms has inhibited the development of high resolution ferroelectric liquid crystal matrix devices. Accordingly, a need remains for a suitable drive waveform for driving a bi-stable ferroelectric liquid crystal matrix display.